Ultrastructure of a reduced developmental cycle (minicycle) in Blastocladiella emersonii

Abstract

Binucleate germlings of Blastocladiella emersonii were induced to differentiate into zoosporangia containing only two zoospores by shifting them from growth medium to dilute phosphate buffer. Samples for both light and electron microscopy were taken every 30 minutes from the time of induction at 3 h after inoculation through zoospore discharge at 7.5 h. The sequence and timing of intracellular structural changes during sporangium formation and zoospore differentiation were estimated from thin sections prepared from each 0.5-h sample. The intrinsic sequence of intracellular change in such “minicycle” cells was found to be similar to that of late log phase cells. It differed from cells induced at the late log phase in that mitosis did not occur after induction, the abundant macrotubular elements characteristics of late log phase cells were absent, and the timing of developmental events was different. Thus, although mitosis may occur, it is not a necessary event in sporulation. Furthermore, the contribution of macrotubular membranes to γ -particle formation, although a normal part of differentiation, is not essential. The absence of the macrotubules may contribute to the longer time required for zoosporangial differentiation in minicycle cells when compared with the “normal” 19-h zoosporangial cycle.